You'll have to read the license agreement for openAI or Google on their tts

Or look into open source tts models with liberal commercial licenses

Generally I'd stay away from the site/project/repo in the OP and just do your own research on this (because the OP site is vibe coded in a sleazy way that's unnecessarily used car salesman-y and doesn't offer anything you couldn't just do better yourself with one codex or chatgpt prompt)

Edit: to be clear there's a lot of different tts providers, they probably all offer tts you can use commercially, you can look at benchmarks for the providers and then the specific TOS

If you put no effort into it, then it’s unlikely you can put any royalty claim on it either? But nobody’s gonna pay for it anyway so don’t worry.